Polyvinyl acetal resins are excellent in strength, facilitate film formation, allow inorganic and organic particles such as pigments to be well dispersed therein, and are excellent in adhesiveness to a surface to be applied to, and thereby polyvinyl acetal resins have been used in various applications such as inks, paints, baking enamels, wash primers, and ceramic green sheets.
Studies have been performed in various fields for further increasing strength and solvent resistance of a polyvinyl acetal resin, and such studies have proposed, for example, a method in which a cross-linking agent is added to a polyvinyl acetal resin and then the mixture is heated to allow the polyvinyl acetal resin to be cross-linked between the molecules.
For example, Patent Document 1 discloses a method in which a polyvinyl acetal resin and an isocyanate resin are mixed with each other and the mixed resin is allowed to be cured. Patent Documents 2 and 3 disclose methods in which a cured film is formed by a thermosetting reaction. Patent Document 4 discloses a method in which hydrogen of a hydroxy group in polyvinyl butyral is substituted by N-methyleneacrylamide and then the polyvinyl butyral is irradiated with light such as electron beam to be cross-linked and cured.
The trouble is that, in these methods, it is required to add a component for triggering a cross-linking reaction, such as a cross-linking agent and a polymerization initiator, at the time of curing or to add it to a polyvinyl acetal resin in advance. Thus, it is difficult to stably store the resins in a state of a solution, and residues of the component for triggering a cross-linking reaction causes staining and degradation. Furthermore, processes get complicated.
In the methods of Patent Documents 2 and 3, a cross-linking reaction is allowed to proceed by thermosetting; however, in the case where a material to be coated with a resin is vulnerable to heat or heating cannot be performed in the process, the methods cannot be employed, or a cross-linking reaction insufficiently proceeds to cause sheet breaking. As disclosed in Examples of Patent Document 3, it requires much time to allow a cross-linking reaction to start without heating. Furthermore, a polyvinyl acetal resin is decomposed in the case of applying electron beam or X rays.
Accordingly, there is a demand for a method which is capable of producing a cross-linked polyvinyl acetal resin having a sufficient degree of cross-linkage and excellent solvent resistance without adding a photopolymerization initiator or a cross-linking agent such as an acryl monomer.
One of the applications of a polyvinyl acetal resin is a material for a thermal transfer ink sheet.
A thermal transfer technique has been conventionally employed as one of the means for forming and recording letters and images. In the thermal transfer technique, a thermal transfer image-receiving sheet is covered with an ink sheet in which a pigment or a dye is dispersed, and then heat is applied above the ink sheet. Thereby, the pigment or the dye is transferred onto the thermal transfer image-receiving sheet to form an image. The thermal transfer technique is broadly divided into a thermal wax transfer technique and a dye sublimation technique. The dye sublimation technique is employed for photo printing, output terminals for medical and analysis devices, photo printing for ID cards, and other purposes. The contemporary digitalizing world expects further improvement of the dye sublimation technique.
Examples of the dye sublimation-type thermal transfer ink sheet include a sheet in which ink layers, such as yellow, magenta, and cyan layers, and a protecting layer for imparting light fastness to a transferred image are successively formed.
Thermal transfer ink sheets for the dye sublimation-type thermal transfer technique have been recently required to be adaptable to high speed printing. In particular, ink layers of a thermal transfer ink sheet are required to be excellent in properties such as heat resistance, weather resistance, and ink retentivity; while a protecting layer is required to contain a highly-strong binder resin component and to have excellent abrasion resistance because it is to be laminated on the outermost surface after transferred onto an image-receiving paper.
In the case where ink layers or protecting layers of a thermal transfer ink sheet is formed by a conventional polyvinyl acetal resin, heat resistance or strength of the sheet is insufficient and thereby the sheet is not adaptable to high speed printing. Furthermore, use of a binder resin having a high glass transition temperature or a binder resin having a high degree of polymerization has been examined for the purpose of increasing the heat resistance and strength. However, these binders cause disadvantages, such as a longer dissolution time of a binder resin and increase in solution viscosity which is controlled upon application to a medium to inhibit easy solution transfer via a pump. Such disadvantages result in reduction in productivity.
Many studies have been conducted from various angles for strengthening a polyvinyl acetal resin used in the aforementioned fields. For example, the following method has been studied; that is, a cross-linking agent is added to a polyvinyl acetal resin and then a mixture is heated, and thereby the polyvinyl acetal resin is allowed to be cross-linked between their molecules.
Patent Documents 1 to 4 disclose methods for curing and cross-linking a polyvinyl acetal resin for dealing with the above problems. The trouble is that, in these methods, it is required to add a component for triggering a cross-linking reaction, such as a cross-linking agent and a polymerization initiator, at the time of curing or to add it to a polyvinyl acetal resin in advance. Thus, it is difficult to stably store the resins in a state of a solution, and residues of the component for triggering a cross-linking reaction causes staining and degradation. Furthermore, processes get complicated.
In the methods of Patent Documents 2 and 3, a cross-linking reaction is allowed to proceed by thermosetting; however, in the case where a material to be coated with a resin is vulnerable to heat or heating cannot be performed in the process, the methods cannot be employed, or a cross-linking reaction insufficiently proceeds to cause sheet breaking. As disclosed in Examples of Patent Document 3, it requires much time to allow a cross-linking reaction to start without heating. Furthermore, a polyvinyl acetal resin is decomposed in the case of applying electron beam or X rays.
Accordingly, there is a demand for a method which is capable of providing a cross-linked polyvinyl acetal resin having a sufficient degree of cross-linkage and high mechanical strength without adding a photopolymerization initiator or a cross-linking agent such as an acryl monomer.
A polyvinyl acetal resin is also used as a binder for an inorganic particle-dispersed paste in which inorganic particles such as electrically conductive powders and ceramic powders are dispersed. An inorganic particle-dispersed paste is formed into a predetermined shape by various printing methods such as screen printing and offset printing. In particular, offset printing can excellently print straight lines and plates for offset printing have high strength, so that production of address electrodes and bus electrodes of plasma displays by offset printing has been tested.
On an offset printing plate used in offset printing, a photosensitive material layer is irradiated with ultraviolet light to develop and form a pattern. Patterns have been finer recently, and accordingly, a resin layer which serves as a protruding portion is required to have high shape retentivity.
A polyvinyl acetal resin excellently adheres to a substrate and a film thereof has excellent strength; however, it cannot be used for pattering. Thus, a polyvinyl acetal resin is tentatively added to a conventional patterning material to achieve good patterning and retentivity. In such a case, although shape retentivity of a protruding portion increases, polyvinyl butyral is difficult to be dissolved in an alkali-washing liquid and thus washing is insufficiently performed, which results in unclear separation between protruding and recessed portions on an offset printing plate.
Studies have been performed from various angles for increasing strength and solvent resistance of a polyvinyl acetal resin. For example, the following method has been studied; that is, a cross-linking agent is added to a polyvinyl acetal resin and then the mixture is heated to allow the polyvinyl acetal resin to be cross-linked between the molecules.
For example, Patent Documents 1 to 4 disclose methods for curing and cross-linking a polyvinyl acetal resin for dealing with the above problems. Patent Documents 2 and 3 disclose curing methods by heat, so that it is not suitable for patterning. The method disclosed in Patent Document 4 requires addition of N-methylene acrylamido group by a post-reaction reaction after production of polyvinyl butyral. This results in problems such as high production cost and residues of a catalyst used in the post-reaction reaction to cause staining or difficulty in stable storage. Furthermore, a curing reaction requires a catalyst, and this results in difficulty in stable storage and discoloration and degradation due to catalyst residues.
Accordingly, there is a demand for a method for providing a polyvinyl acetal resin which excellently adheres to a substrate, forms a film having high strength, and is cross-linkable by application of ultraviolet light without a catalyst.    Patent Document 1: Japanese Kokai Publication No. 2006-156493 (JP-A 2006-156493)    Patent Document 2: Japanese Kohyo Publication No. 2006-522863 (JP-W 2006-522863)    Patent Document 3: Japanese Kohyo Publication No. 2006-523754 (JP-W 2006-523754)    Patent Document 4: Japanese Kokoku Publication No. H07-14973 (JP-B H07-14973)